Frequency discriminator



M. E. BOND FREQUENCY DISCRIMINATOR Filed June 11, 1943 ATTORNEYS INVENTOR. MARION E'. BOND mwbi: 2

July 23, 1946.

Patented July 23, 1946 FREQUENCY mscamnmr'oa Marion E. Bond, Chicago, Ill.', assignorf to'Galvin Manufacturing Corporation, Chicago, 111., a corporation of Illinois Application June 11, 1943, Serial No. 490,467 (01. 250-27 '7 Claims. 1

The present invention relates to frequency discriminators and more particularly to improvements in frequency discriminating networks of the character utilized in frequency modulated radio receiving systems for detecting the modulation components of a received frequency modulated carrier.

Conventional frequency discriminators, as used ahead of the audio frequency channel of a frequency modulated receiver for detecting the modulation components of a modulated intermediate frequency carrier, conventionally comprise a resonant circuit tuned tothe intermediate'frequency and designed'to respond to frequencies which exceed or are less than the intermediate frequency by not more than a predetermined amount. In the usual arrangement, the signal modulated intermediate frequency carrier is introduced into the discriminating network by tapping the winding of the fixed inductance forming a part of the resonant circuit at its electrical mid-point, and connecting or coupling the output circuit of the preceding amplitude limiter tube to the center tap'of the inductance element. The capacitance coupling thus provided is supplemented by inductively coupling the inductance element of the discriminator to the output inductance element of the preceding limiter stage. The tuning of the resonant circuit to the desired intermediate frequency is universally accomplished by using an adjustable condenser connected in shunt with the inductance element. The adjustable resonant circuit thus formed is combined with additional circuit apparatus which includes a pair of rectifying paths and has the function of producing an output voltage which varies in magnitude at the signal frequency rate in accordance with departures of the resonant circuit exciting frecriminator it is essential that the resonant cirr 2 cuit be accurately and precisely tuned to the center intermediate frequency and that it be accurately and precisely balanced to produce no output voltage at this frequency. It is also necessary that the output voltageof the circuit accurately and faithfully respond to given departures in frequency from the intermediatefrequency in either direction. These ends are, difficult to achieve with typical prior art arrange: ments, because of the difficulties inherent in accurate condenser tuning of, the resonant circuit and the difliculties inherent in accurately center tapping .inductances at "their electricallmide points in commercial quantities. I It-is .an object of the present invention, there'- fore, to provide an improved frequency discrim inator which is simple in arrangement and may be readily reproduced in production quantities to have an exact and predetermined operating characteristic. V

It is another object of the present invention to provide an improved discriminator which may be easily and accurately adjusted to produce no output voltage at a particular frequency and to produce an output voltage which accurately conforms both in polarity and magnitude to deviations in the exciting frequency from the particu lar frequency.

It is a further object of vide a frequency modulated radio receiving system which includes an improved and exceedingly simple arrangement for accurately and faithfully detecting the audio frequency components of a received frequency modulated carrier.

It is still another object of the invention to provide an improved discriminator or network wherein condensers of fixed capacitance value and an adjustable inductance element are utilized to form the tuned circuit which determines the frequency response characteristic of the network. r

According to another object of the invention, an inductance element of the adjustable permeability' type is utilized to tune the resonant frequency of the discriminator network to the the network at the junction point between .twb

the invention to prostood-by reference to the following specification;

taken in connection with the accompanying 7 drawing, in which:

Fig. 1 represents a frequency grnodulated radio receiving system having incorporated therein an* improved discriminator network characterized by the features of the invention briefly referred to above;

, Fig.2 is'an equivalent circuit of the frequency discriminator. network shown in Fig. 1;

Fig.3 is a graph illustrating the difierent response characteristicswhich are obtained for the discriminator when the circuit elements of the discriminator are arranged indifferent manners and at least one of the circuit constants of the. networkis varied; and 2 Fig. 4 isa circuit diagramlillustratinga modification of, the receiving system shown in Fig. 1. r

Referring nowfmore-particularly 'to Fig.1 of the drawing, there .isrepresented, partially schematically, a frequency-modulation. receiving.

system. embodying the frequency discriminator of the invention. v1 This receiving. system includes the'following units, connected infcascade, in the .orderi named z.. a tunablefantenna' circuit 1 8, a

V tunable radioifrequency amplifier 20,"a first mixer onconverter. 1stagel2l, a first intermediate fre-. quency, amplifier i, 22,, a second mixer or [converter stage .23, a second intermediate frequency amplifier 24, a first limiter 25, a second limiter 26, a frequency discriminator 2-1, anaudiofrequency amplifier 28, anda loudspeaker 29, all connected in cascade in theorder named. l The frequency discriminator 2Tto which the present invention more specifically relates 00mlprises acircuit 81, a pair of'diode rectifier tubes 88 and 89, thespace current paths of which are .respectively shunted by load resistors 90 .and 9| a radio frequency by-pass condenser 93, having substantially negligibleimpedance to frequencies of the order of the second intermediate frequency, and a-stabilizing condenser 92. More specifically, the resonant circuit 81 comprises a pairof series connected condensers 811) and 81c which are shunted byian adjustable inductance element 81a. Preferably the last mentioned elementis of the/ variablewpermeabiiity type being provided with'an adjustable powdered ferrous .metal'core, theposition of which may be changed to alter the inductance of the, element within the desired.

limits. The circuit constants of the resonant .cire

cuit 8! together with .the associated capacitances including condenser 92 are o chosen'that this circuit is tuned to the nominal intermediate carrier frequency and. sufiiciently broadly tuned so that all desired signal components of ,a frequency modulated carrier appearing in the'second'intermediatefrequency channel 24, 25, 29 maybe'detectedand impressed upon the input circuit of the audio: amplifier 28. The voltage appearing :across the output'side of the second limiter 26 is impressed upon the discriminator network 21 through a coupling condenser 86 which is connected at one side thereof to the junction point ,Bld between the two condensers 81b and 810.

Actually, the condenser 86 provides the only coupling path between the output side of the limiter 26 and the discriminator 21, a grounded conductive shielding housing being provided for enclosing the circuit elements. 87a, 81b and 970 so that mutual inductive coupling or stray capacitance coupling between the discriminator and the preceding limiter is absolutely eliminated. Audio frequency voltages detected'through operation of thediscriminator 21 appear across the condenser "93 andare impressed uponthe input side of the audio frequency amplifier 28 through a coupling circuit whichincludes radio frequency decoupling I resistor 94. an audio'freque'ncy filter comprising f the'resistor;-95 and condenser 96, an audio frequency coupling condenser 92. and a voltage dividing, network comprising the two resistors 98 and 199 and. a direct current blocking 'condenser llll. It will be understood in this regard that the proportion of the available audio frequency voltage appearing across the series connected resistors 98-and l which is impressed upon the input" circuit of the audio frequency amplifier 28, is determined by the setting of the wiper 99 along the resistor 98. d l

The operation of the above described signalreceiving system, as a whole, will .be clear to those 'skilledin the art from the foregoing'description. In brief, a frequency-modulated carrier signalintercepted by the antenna circuit .19 is selected and amplified in theuriit '18,- further selected. and amplified in the I radio-frequency amplifier 29, converted in the first miXeriZi into a frequency-modulated intermediateefrequency waveiwhich is amplified and selectediri the amplifier 22, converted in the second mixer 23 vtof'a second frequency-modulated, intermediate' frequency signal .of a lower frequency, further selected and amplified in the amplifier 2 3, limited in limiters 25 and 26 and applied through the coupling condenser 86 to' the frequency 'dis-, criminator 2'! embodying the invention, from which it is applied through the audio-frequency amplifier 28 to the loud speaker 29 for reproduce tion. u

Referring now more particularly to theoperation of the discriminator 21, it will be noted that this circuit is essentially a four arm, four terminal bridge circuit of the charactershownin Fig. 2 of the drawing. In this bridge circuit the capacitances C1 and C2 are of equalvalueand represent the condensers 81b and 910, respec tively, the capacitor C3 represents the capacitance of the diode .88, the capacitor C4 represents the combined capacitance of the diode 89 and the shunting condenser 92 and the inductance L rep resents the tuning inductance 81a. The diode rectifiers 88 and 89 are thus efiectively individually coupled across'the two adjacent capacitance arms C3 and C4, respectively, while the resistors 90 and 9| individually "shunt the rectifiers88 and 89;, respectively. Since the load resistors 90 andv 9| have impedances far in excess of the impedances' of the capacitors Q3 and C4 at the frequencies; involved, they may be. neglected in analyzing the circuit; Again; the capacitance of the condenser 93 is so much greater than that of either the capacitor (her the capacitor G g-that this condenser may also be neglected in analyz ing' the circuit; The signal modulated carrier voltage E, which appears at the output side of the limiter 26, is applied to the circuit solely between the terminals D and F, and the voltage appearing at the output side of the discriminator is the difference between the absolute .values of the voltages to ground at the terminals A-and B. From an examination of the bridge, it will be understood that if C1 equals G2, which it does, and C3 equals C4, such that the bridge is balanced, the currents i1 i2 respectively traversing the capacitors C1 and C2 are equal so that equal voltage drops c1 and e2 appear across these capacitors; Accordingly, nodiflerence between the voltages to ground is developed at the points Aand B, regardless of the frequency of the exciting voltage E. In the actual circuit, however, the capacitance C3 is less'thanthe capacitance C4 by an amount equal to the capacitance value of the condenser 92 such that thebridge is unbalanced. Accordingly, during excitation of the circuit by the voltage E, the current i2 exceeds the current '2'1 so that a current 2: is caused to flow through capacitors C1 and C2 of the inductance element L. The magnitude of this current obviously depends upon the reactive impedance of the inductance L at the particular frequency of excitation and the direction of currentflow is such that the voltage drop across the capacitance C1 is enhanced and that across the capacitance C2 is decreased. It will be understood, therefore, that by suitably proportioning the impedance of the inductance L relative to the reactive impedances of the capacitances C1 and Czat a particular' center frequency, to establish a given reiationship between the currents i1, i2 and is the absolute voltages between the points A and Band ground become equal. In their relationship to each other,

however, these voltages are out or phase so'that a difference voltage'actually exists betweenthe points A and B. This difference voltage is, of

course, equal to the vector sum of the absolute" voltages from the points A and B to the ground between the cathode of the'diode 38 and around.v

As the exciting voltage for the resonant circuit Bl'is increased above the center frequency, due to the signal modulation thereof at an audio rate, the reactive impedances of the circuit constants change to alter the relative magnitudes of the currents 2'1, 2'2 and is so that the voltage from the point A to ground exceeds that between the point B and ground. Accordingly a voltage which is positive with respect to ground is producedgbe tween the cathode of the diode 5'3 and ground. If, on the other hand, the exciting frequenc-yfcr the circuit 81 is'decreased below the center frequency, the reactive: impedances of the circuit constants change to alter the relative magnitudes of the currents i1, i2 and is so that the voltage between the point B and ground exceeds that between the point A and ground- As a result an output voltage which is negative with respect to ground is produced between the cathode of the diode 83 and ground.

' More specifically considered the voltage between the point A and ground, mathematically expressed, is as follows:

E=exciting voltage Q=the Q oithe inductance element 87a w=21rf Y EC =C +CCa+GC4+C3C4 The expression forthe voltage between the point B and ground is similar, viz: v

. a 2 Vb EC a 7{(C+ on no? 2c n {q .L w

By evaluating the above voltages for different values of ma circuit having"selectedcircuit constants and excited at a particular voltage, and plottingth difference voltage'therebetween, it maybe shown that at the center resonant. fre quency of the discriminator network 21 the ,difference voltage is zero and that'this difference voltage varies linearly with'frequency changes above and below the center resonant frequency.

. voltage is accurately reproduced across the condenser 93 at the output 'side'of the discriminator 21. To consider somewhat more fully the action of the condenser 92 in stabilizing the'operation of the discriminator network 21," it maybe pointed out that-if theim-pedances of the'four legs of the bridge circuit are perfectly balanced, changes in the exciting frequency will not'produce the desired differences of potential between the terminals A and B. By providing the condenser 92 connected in the manner illustrated, however, thereby to insure that the over-allcapacitance between the lower terminal B of the resonant circuit 81 and ground exceeds that between the upper. terminal A of this circuitand ground, the desired circulating current within the resonant circuit will always be produced toinsure stability of circuit operation. In this regard it is pointed outthat the unbalancing or stabilizing condenser-.92 may be connected either between j the lower terminal of the resonant circuit. 81 and minal of the resonant circuit and ground.

It will be understood fromthe aLbbve-explana tion that the inventionzisnot limited to any particular range of frequency detection, although I it is particularly applicable; to'the detectionrof: signal frequencies within a band embracing the audio frequency range. In order to'illustrate the relative magnitudes of the principal circuit n elements of the discriminatorsn'etworkl 'Lwhen designed in accordance with the present invention, the following approximatevalues of its elements, together with other pertinent-information, are given for a particular system'of the character shown in Figs. 1 and 2 of thedrawing'.

Center intermediatelfrequency A; r I I megacycles Condenser"87b micro microfarads 100 erfiic r 1 Condenser 93; do 100 Condenser 92-; do 10 Resistor 90 ohms 220,000 Resistor. 91 do' 220,000 Inductance element 87a adjustable from Q imicro"henries 50 Resistor 94 7 Ohms 100,000 Resistor 95 do 470,000 Condenser 97' microfarads .01 Condenser 96 do 1Q .01" Condenser 10 1..- do .01 Resistor 98 1;a ohms 500,000

. response haracteristics shown in Fig. 3 of the drawing are typically representative of the improved discriminator 21. when provided with circuit'qelements having the approximate values indicated,,but with diiferent capacitance values for the condenser 92. In the graph illustratedin this figure,positive and-negative frequency deviations. fromthe center intermediate frequency. of 2.5l5 megacycle's, as graduated in positive vand negative frequency steps .of ten kilocycleseach, are plotted as a functionof positive and negative voltage output across the. condenser 03. 7 More specifically, the response curves A, B and C or this graph representaccurately reproduced test curvesof a discriminator network of the character described when provided with a condenser 92 connected between the lower terminal of the resonant circuit 81 and ground. 'By a comparison-jof these curves, it will benoted that as the .value 'of this condenser is increased from a value .of 5 micro microiarads upward toa value of 9 micro microfaradsthe linear portion of the resulting response characteristic of the network is substantially increased. It will also be seen from anexamination of. these curves that, as the excitinggifrequency of .the network is increased above the. center intermediate frequency at which the resonant circuit8l is tuned; an output voltage On' the otherhand, when the exciting frequency changes in the exciting frequency ofthe reso-- nant circuit 81.

The curve D, as'shown in Fig. 3'of the draw-'- ing, is an accurate. reproduction of the same circuit when equipped with a condenser'92, of .5 v

micro microfarads. connected between the upper terminal of the resonant circuit 81 and ground. Bya'comparison-of this curye with the three curves A, B and C, it willbe noted'that the polarity of the output voltage produced in response H to increases and decreases in the :exciting' frequency of the resonant circuit'8l is reversed from that which obtains when the condenseraSZ isconnected between the lower terminal of the resonant circuit and ground. This is due to the fact,

7 noted-above, that when the position of this conwhich is positive with respectto ground is produced at the output side of'the discriminator.

denser in the network is changed from a'connection between-the lower terminal of the resonant circuit and ground to a connection between the upper terminal of this resonant circuit and ground, the direction of circulating current flow through the elements of the resonant circuit'B'I is reversed. r

Ifdesired, one rectifying section of'the' improved discriminator 21 may be combined with the audio frequency amplifier 28 in the manner illustrated'in Fig. 4 of the drawing, whereinref= erence characters corresponding'to those used in Fig. 1 identify the same circuit elements. From anexamination of the Fig. 4 arrangement, it will be seen that the diode section of the tube 89 is utilized as one of the rectifying paths of the discriminator, and that the cathode, anode and three grids of the tube are used to amplify the audio frequency voltage which is developed between. the wiper 99 and ground during reception of a selected s gnal. mitted to the loud speaker'29 for reproduction through a coupling transformer I39. The manner in which the audio section of thetube is blocked under the control of the muting oscillator 33 and mode of operation of the discriminator 2 1 are exactly the same as explained herein with reference to the system, shown in Fig. 1. In

fact, the circuit of Fig. 4 may be directlysubstituted for the discriminator 21 and the audio frequency amplifier 28 in the system of Fig. 1.

From the fore oing explanation with reference to the'discrimi'nator 21, itwill be understood that this circuit'may be accurately reproduced according to a preoonceived design to havethe exact desired operating characteristics. Thus,'in the manufacture. of this circuit, it'is'unne essary to utilize a tuning inductance element 81a having a center tan precisely located at the el ctrical by utilizing available commercial inductance elements 81a of the variable permeability type, a resonant circuit 81 may be provided which is precisely' and accurately tuned to the desired frequency such that no output voltage is produced 7 across the condenser 93 when the discriminator network is excited at the center intermediate frequency. This is aniimportant consideration in the automatic controlof the reactance-modula- This. audio voltage is transtortu'be 38 to maintain the system faithfully responsive to a selected signal, as will appear from the explanation which follows.

More generally considered, the discriminator network 21 is substantially more stable and less afiected by stray capacitance coupling paths than the discriminators previously available in the prior art. Thus, the tuning slug of the variable inductance 81a may be connected to ground so that body capacity'efiects do not affect the circuit constants of the network in tuning the res-' onant circuit 81 toa desired center frequency. This makes it possible to easily tune the network with precision accuracy to the exact desired center frequency, so that the network is perfectly balanced in so far as the relationship between frequency deviations and voltage output is concerned. As contrasted with this improved arrangement, a conventional discriminator utilizes an adjustable tuning condenser having its adjustable plates above ground potential. When, therefore, a tuning tool is brought into proximity to the adjusting shaft, stray capacitance coupling to the elements of the tuned discriminator circuit is provided which makes exact tuning of the circuit to the desired center frequency practically impossible.

Also, by shielding the elements of the circuit 81 and the elements used at the output side of the limiter 26, thereby to eliminate mutual inductive coupling between the discriminator and the preceding limiter 26, and by using only the capacitive coupling of the condenser 85 between these stages, the discriminator network may be much more accurately manufactured and adjusted to operate in accordance with given design specifications. Moreover, by capacitively coupling the preceding limiter 26 into the discriminator 21 through the condensers 86, 81b and 810, stray capacitances are minimized and the effects of those remaining may be more easily compensated for in the design and adjustment of the network. A further advantage of the disclosed discriminator 21 resides in the fact that it may be more easily and accurately compensated for temperature variations by making suitable provision for adjusting the tuning slug of the inductance element 81a in response to changes in the temperature to which the elements of the network are exposed.

While one embodiment of the invention has been disclosed, it will be understood that various modifications may be made therein, which are within the true spirit and scope of. the invention.

I claim:

1. In a frequency discriminator network which is adapted to be excited from a voltage source of variable frequency, a pair of series connected condensers, a two-terminal adjustable inductance shunting said series connected condensers to tune said network to a particular center frequency, means including a first rectifying element for bridging one of said condensers across said source, means including a second rectifying element for bridging the other of said condensers across said source, load resistors respectively shunting said rectifying elements, and means providing unequal capacitance across said rectifying elements to unbalance said network.

2. In a frequency discriminator network which is adapted to be excited from a voltage source of variable frequency, a pair of series connected condensers, a two-terminal adjustable inductance shunting said series connected condensers to tune said network to a particular center frequency,

means including a first rectifying element for bridging one of said condensers across said'sourc'e, means including a second rectifying element for bridging the other of sai'dcondensers across said source, load resistors respectively shunting said rectifying elements, and an unbalancing capacitance element connected in shunt with one of said load resistors. I

3. In combination with a voltage source of variable frequency, a pair of rectifying paths bridging said source and each including a rectifying element and a condenser arranged in series circuit relationship, means comprising a twoterminal tuning inductance shunting said condensers in series, means providing unequal capacitance across said rectifying elements for varying the relative magnitudes of the voltages appearing across said rectifying elements in accordance with deviations in the frequency of said voltage source from a particular value, load resistors respectively shunting said rectifying elements, and a direct current path serially including said resistors and said inductance so that a voltage is produced thereacross which varies in accordance with the extent of deviations of the frequency of said voltage source from said particular frequency.

4. In combination with a voltage source of variable frequency, a pair of rectifying paths bridging said source in parallel and each including a rectifying element and a condenser arranged in series circuit relationship, means comprising a two-terminal inductance shunting said condensers in series and a condenser shunting one of said rectifying elements for varying the relative magnitudes of the voltages appearing across said rec- -tifying elements in accordance with deviations in the frequency of said voltage source from a particular value, load resistors respectively shunting said rectifying elements, and a direct current to be excited from a variable-frequency signal source comprising, a four-arm capacitance bridge circuit including connections for excitation from said source across one diagonal thereof, a twoterminal adjustable inductor connected across the the other diagonal of said bridge circuit to tune said bridge circuit to the mean value of said variable frequency, a pair of rectifying devices individually coupled across two adjacent capacitance arms of said bridge forming a branch in parallel to said inductor, a pair of resistors individually shunting said rectifying devices, and load impedance means coupled to said resistors for deriving the differential of the unidirectional signals developed thereacross by said rectifiers,

6. A frequency discriminator network adapted to be excited from a variable-frequency signal source comprising, a four-arm capacitance bridge I circuit including connections for excitation from said source across one diagonal thereof, a two-terminal adjustable inductor connected across the other diagonal of said bridge circuit to tune said bridge circuit to the mean value of said variable frequency, the adjacent capacitance arms of at least one of the bridge branch circuits in parallel to said inductor having unequal capacitance values, a pair of rectifying devices individually coupled across two adjacent capacitance arms of said bridge forming a branch in parallel toisaid inductor; a pair of resistors ind1 rvidually jshunting :said rectifying devices, and

means for deriving the difierential'v of the unidirectional signals developed by said rectifiers'.

V 77; A frequency'discriminator network adapted to be excited'from a variable-frequency signal source comprising, a four-arm capacitancebridge circuit including connections for excitation from said source across one diagonal thereof, a twoterminal inductor including an adjustable ferromagnetic core for adjusting the inductance value thereof, said inductor being connected across the 1 iulirtroiq E. 301 115.. I 

